The use of fine grain photo emulsion for the preparation of a reflective laser recording material was first disclosed by J. Drexler in U.S. patent application Ser. No. 131,288, generally corresponding to German Offenlegungsschrift No. 3,002,911. In that application, a processed black filamentary silver emulsion was converted to a reflective non-electrically conductive recording medium by heating at a temperature in the range of 250.degree. C. to 330.degree. C. in an oxygen containing atmosphere until the surface developed a reflective appearance. This laser recording material worked effectively with lasers of visible wavelengths, but its recording sensitivity fell by a factor of three for semiconductor lasers, which generate light in the near infrared at about 830 nm. The high temperatures of the process preclude the use of plastic film substrates commonly used for photographic films.
In U.S. Pat. No. 4,278,756 to E. W. Bouldin and J. Drexler, a reflective data storage medium is described. A reflective silver recording layer is derived from silver-halide emulsion through a silver diffusion transfer process. No heating was required to create the reflective surface; reflectivities up to 25% of green light were achieved. However, the recording sensitivity of this material was less than that of the process described in the aforementioned U.S. patent application Ser. No. 131,288, which yielded reflectivities up to 17%.
In U.S. Pat. No. 4,278,758 to J. Drexler and E. W. Bouldin, a reflective medium was disclosed derived from a silver-halide emulsion through a diffusion transfer process. In this medium the recording sensitivity at green laser wavelengths was greatly improved over that described in U.S. Pat. No. 4,278,756 and even somewhat higher than that achieved by the medium described in the aforementioned U.S. patent application Ser. No. 131,288. It was necessary, however, to add an annealing step at a temperature of 250.degree. C. and above to achieve the desired results. Although the recording sensitivity was very good with a green laser at 514 nm and with a red laser beam at 633 nm, it fell off by a factor of three when the laser wavelength was increased to 830 nm.
In U.S. Pat. No. 4,284,716, Drexler and Bouldin addressed the problem of retaining recording sensitivity in the red and infrared wavelengths while retaining use of common plastic substrates through avoidance of the thermal annealing step. This was achieved by combining the two known forms of chemically reduced silver metal, spheroidal and filamentary, at the surface of the reflective recording material.
The process by which this was achieved involves use of fine grain photographic emulsion which is given a weak light exposure and then treated in a strong chemical developer. This developer contains no silver-halide solvent and thus proceeds through chemical development or "direct development" to produce amorphous filaments of silver metal, which are highly absorptive of red and near-infrared light. The photographic emulsion is then briefly contacted with a chemical fogging solution which is a strong silver ion reducer. Small silver nuclei are now created at the top of the emulsion surface because of the non-penetrating nature of the fogging solution's solvent and the briefness of the contact. When the photographic emulsion is immersed in a monobath containing a developing agent and a silver-halide solvent, silver ions from throughout the emulsion are transported to the thin layer of nuclei at the emulsion surface and there reduced to silver metal by the developing agent. Silver metal reduced from a solution onto nuclei is a process known as "solution physical development." The silver formed this way is often in the form of regular octahedrons or spheroids. When these spheroids are large and/or numerous enough, they can grow into each other to form agglomerates which by virtue of their regular faces or high volume concentration reflect visible and near-infrared light. This invention, then, made use of the absorptive filamentary silver in an intimate dispersion with reflective spheroidal silver to produce a sensitive laser recording material that could be prepared through room temperature chemistries. This media is more sensitive in the green and blue than it is in the red and infrared.
In order to increase recording sensitivity, others have previously devised recording media using dyes which are absorptive of the recording wavelengths. For example, in U.S. Pat. No. 4,230,939 to de Bont et al. an auxiliary layer is provided containing an organic dye. See also "Infrared Dyes for Optical Storage", V. B. Jipson and C. R. Jones, J. Vac. Sci. Technology 18(1) Jan/Feb 1981.
An object of the present invention was to devise a more sensitive reflective direct read after write (DRAW) laser recording and data storage medium. Another objective is to devise a laser recording medium which is more sensitive at particular wavelengths. A further objective is to devise a general purpose highly absorptive underlayer for many types of laser recording media. Another object is to reduce the manufacturing process to fewer steps, with smaller cost in terms of time and material, and with a greater degree of process uniformity in creating red and infrared reflectivities in the 20-50% range.